1,721,165 research outputs found
Learning universality and scaling from simple deposition models
No full textWe use deposition models of kinetic roughening of a growing surface to introduce the concepts of universality and scaling and to analyze the qualitative and quantitative role of different parameters. In particular, we focus on two classes of models where the deposition is accompanied by a local relaxation process within a distance δ. The models are in the Edwards-Wilkinson universality class, but the role of δ is nontrivial
'Some words about death...'. Observations concerning death of some psychiatric patients
No full textPsychiatric institutions, as also medical ones, appear particularly afraid of meeting death, because of its symbolic implications. Beside a higher mortality among mental patients, when compared to the general population, stands the empirical observation that psychiatrists are only seldom engaged in treating dying patients. On the basis of these assumptions, the authors examine briefly some emblematic case histories of patients who died during admission or soon after their discharge from the General Hospital Psychiatric Ward of Pavia, Italy. They underline some peculiarities of dying in psychiatry and consider what happened among the staff before the death of those patients
Quasideterministic dynamics, memory effects, and lack of self-averaging in the relaxation of quenched ferromagnets
No full textWe discuss the interplay between the degree of dynamical stochasticity, memory persistence, and violation of the self-averaging property in the aging kinetics of quenched ferromagnets. We show that, in general, the longest possible memory effects, which correspond to the slowest possible temporal decay of the correlation function, are accompanied by the largest possible violation of self-averaging and a quasideterministic descent into the ergodic components. This phenomenon is observed in different systems, such as the Ising model with long-range interactions, including the mean-field, and the short-range random-field Ising model
One Dimensional Phase-Ordering in the Ising Model with Space Decaying Interactions
No full textThe study of the phase ordering kinetics of the ferromagnetic one-dimensional Ising model dates back to 1963 (R. J. Glauber, J. Math. Phys. 4, 294) for non conserved order parameter (NCOP) and to 1991 (S. J. Cornell, K. Kaski and R.B. Stinchcombe, Phys. Rev. B 44, 12263) for conserved order parameter (COP). The case of long range interactions J(r) has been widely studied at equilibrium but their effect on relaxation is a much less investigated field. Here we make a detailed numerical and analytical study of both cases, NCOP and COP. Many results are valid for any positive, decreasing coupling J(r), but we focus specifically on the exponential case, Jexp(r) = e-r/R with varying R> 0 , and on the integrable power law case, Jpow(r) = 1 / r1+σ with σ> 0. We find that the asymptotic growth law L(t) is the usual algebraic one, L(t) ∼ t1/z, of the corresponding model with nearest neighbouring interaction (zNCOP= 2 and zCOP= 3) for all models except Jpow for small σ: in the non conserved case when σ≤ 1 (zNCOP= σ+ 1) and in the conserved case when σ→ 0 + (zCOP= 4 β+ 3 , where β= 1 / T is the inverse of the absolute temperature). The models with space decaying interactions also differ markedly from the ones with nearest neighbors due to the presence of many long-lasting preasymptotic regimes, such as an exponential mean-field behavior with L(t) ∼ et, a ballistic one with L(t) ∼ t, a slow (logarithmic) behavior L(t) ∼ ln t and one with L(t) ∼ t1/σ+1. All these regimes and their validity ranges have been found analytically and verified in numerical simulations. Our results show that the main effect of the conservation law is a strong slowdown of COP dynamics if interactions have an extended range. Finally, by comparing the Ising model at hand with continuum approaches based on a Ginzburg–Landau free energy, we discuss when and to which extent the latter represent a faithful description of the former
Universality in the time correlations of the long-range 1d Ising model
No full textThe equilibrium and nonequilibrium properties of ferromagnetic systems may be affected by the long-range nature of the coupling interaction. Here we study the phase separation process of a one-dimensional Ising model in the presence of a power-law decaying coupling, J(r)=1/r1+σ with σ>0, and we focus on the two-time autocorrelation function C(t,tw)=⟨si(t)si(tw)⟩. We find that it obeys the scaling form C(t,tw)=f(L(tw)/L(t)), where L(t) is the typical domain size at time t, and where f(x) can only be of two types. For σ>1, when domain walls diffuse freely, f(x) falls in the nearest-neighbour (nn) universality class. Conversely, for σ≤1, when domain walls dynamics is driven, f(x) displays a new universal behavior. In particular, the so-called Fisher-Huse exponent, which characterizes the asymptotic behavior of f(x)≃x−λ for x≫1, is λ=1 in the nn universality class (σ>1) and λ=1/2 for σ≤1
Quasideterministic dynamics, memory effects, and lack of self-averaging in the relaxation of quenched ferromagnets
No full textWe discuss the interplay between the degree of dynamical stochasticity, memory persistence, and violation of the self-averaging property in the aging kinetics of quenched ferromagnets. We show that, in general, the longest possible memory effects, which correspond to the slowest possible temporal decay of the correlation function, are accompanied by the largest possible violation of self-averaging and a quasideterministic descent into the ergodic components. This phenomenon is observed in different systems, such as the Ising model with long-range interactions, including the mean-field, and the short-range random-field Ising model
Vita nei lacioni e dintorni, 3
No full textL’indagine è stata condotta in vari biotopi astatici (lacioni) con raccolte faunistiche di elementi del plancton e del benthos, che hanno permesso di produrre un elenco faunistico non esaustivo. Una parte più dettagliata del monitoraggio, comprendente dati sia qualitativi sia quantitativi, è stata svolta sulla componente faunistica a crostacei entomostraci, permettendo di delinearne le dinamiche stagionali anche in connessione con le tipologie idrologiche dei diversi lacioni. Lo studio pone anche in relazione le caratteristiche pluviometriche medie della riserva con le differenti fasi di sviluppo della fauna acquatica di questa acque astatiche
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